Why we cannot grow a human arm.

There are several significant issues that prevent us from growing a human arm now, or within the next 10-20 years. From a tissue engineering perspective, while we can grow many of the components necessary for construction of a human arm, we can only grow them in relatively small volumes, and when scaled up to large volumes we lack the ability to develop adequate blood/nerve supply. From a genetic engineering perspective, we will probably never be able to turn on the specific genes necessary to "grow an arm" unless it is attached to a fetus and this presents enormous ethical issues related to farming of human organs and structures. Perhaps the most daunting problem facing the transplantation of a tissue engineered or transplanted arm is that of re-innervation of the structure. Since the sensory and motor nerve cells of the arm are located outside of the structure, re-innervation requires those nerves to regenerate over relatively large distances to repopulate the nervous system of the arm. This is something with which we have had little success. We can grow repair parts, but "growing an arm" presents too many insurmountable problems. The best we could possibly do with tissue engineering or genetic engineering would be the equivalent of a fetal arm and the technical problems, costs, and ethical hurdles are enormous. A more likely solution is a functional, permanent, neuroelectronically-controlled prosthesis. These are nearly a reality today.